Export as loop wrap-around

Background: If you plan to loop a song/audio snippet with (long) reverb you cannot export as loop as that will cut out the reverb.

Proposal: Render the last bar/two bars or something, then clip out that audio and place that/those bars on top of the first bar(s).

Start by locating the export-as-loop entry point and the rendering and clip-boundary logic; the issue does not name files or tests. Reproduce the problem with a looped audio snippet using long reverb, then determine how the rendered tail can wrap onto the loop start without being cut off. Done means the exported loop preserves the reverb across the loop boundary.

Written by the indexing model from the issue text.